1. Field of the Invention
The present invention relates in general to the economy of energy in electronic ballast circuits of the self-excited generation type, and more particularly to an electronic ballast circuit for discharge lamps in which an inductance of a ferrite core is used to supply a stable transistor base current with no use of a separate direct current (DC) voltage source.
2. Description of the Prior Art
Generally, a high voltage is required to light discharge lamps such as fluorescent lamps in an initial state. After the lighting of the discharge lamps, an amount of current flowing through the discharge lamps must be controlled to maintain an illumination of the discharge lamps constant. This lighting characteristic of the discharge lamps requires the use of a ballast circuit.
Referring to FIG. 1, there is shown a circuit diagram of a conventional electronic ballast circuit for discharge lamps. As shown in this drawing, the conventional electronic ballast circuit comprises a rectification circuit 1 for full wave-rectifying and smoothing an input alternating current (AC) voltage to output a DC voltage, a drive voltage supply circuit 2 for receiving the DC voltage from the rectification circuit 1, limiting an amount of current to a transformer T3 and supplying an initial drive voltage to an inverter 4, and a base driving circuit 3 for dropping a voltage induced in a third winding L33 of the transformer T3 and the input AC voltage, full wave-rectifying and smoothing the dropped AC voltage and outputting the resultant DC voltage as a base drive voltage.
The inverter 4 is adapted to drive the transformer T3 in response to the DC voltages from the rectification circuit 1 and the base driving circuit 3.
A current control circuit 5 is also provided in the conventional electronic ballast circuit to stabilize a current induced in the transformer T3 and then supplied to the discharge lamps.
The operation of the conventional electronic ballast circuit with the above-mentioned construction will hereinafter be described.
Upon application of the input AC voltage to the rectification circuit 1, the input AC voltage is full wave-rectified by diodes D1-D4 and then smoothed by a condenser C1. As a result, the DC voltage is supplied from the condenser C1. The DC voltage from the condenser C1 is supplied to a base of a transistor Q1 in the inverter 4 through a turning-on resistor R1, thereby causing the transistor Q1 to be turned on. Because of the turning-on of the transistor Q1, a current flows through a current limiting transformer T1 in the drive voltage supply circuit 2, a primary winding L31 of the transformer T3 and the transistor Q1.
At this time supplied to a base of a transistor Q2 in the inverter 4 is a high voltage resulting from a voltage induced in the third winding L33 of the transformer T3, whereas a low voltage is supplied to the base of the transistor Q1. As a result, the transistor Q2 is turned on, while the transistor Q1 is turned off.
At that time that the transistor Q2 is turned on and the transistor Q1 is turned off, a current flows through the current limiting transformer T1, a secondary winding L32 of the transformer T3 and the transistor Q2, thereby causing a voltage to be induced in the third winding L33 of the transformer T3 in the opposite direction to that in the case where the primary winding L31 of the transformer T3 is driven as mentioned above. As a result, a high voltage is supplied to the base of the transistor Q1 and a low voltage is supplied to the base of the transistor Q2. In result, the transistor Q1 is turned on, while the transistor Q2 is turned off.
In this manner, the voltage induced in the third winding L33 of the transformer T3 is inverted in polarity whenever the primary winding L31 and the secondary winding L32 of the transformer T3 are alternately driven as the transistors Q1 and Q2 are alternately turned on. With the above operation repeated, a voltage is induced in a fourth winding L34 of the transformer T3 in proportion to the voltages induced in the primary winding L31 and the secondary winding L32 of the transformer T3. The voltage from the fourth winding L34 of the transformer T3 is supplied to the discharge lamp through condensers C4 and C5, resulting in a discharging operation.
By the way, in order to drive the transistors Q1 and Q2 more stably, a voltage is supplied to the bases of the transistors Q1 and Q2 separately from the voltage induced in the third winding L33 of the transformer T3. This separate voltage is supplied from the base driving circuit 3. Namely, in the base driving circuit 3, the input AC voltage is dropped by a predetermined level by a voltage dropping transformer T2. The dropped AC voltage is full wave-rectified by diodes D5-D8 and then smoothed by a condenser C2. As a result, the DC voltage is supplied from the condenser C2. The DC voltage from the condenser C2 is supplied to the bases of the transistors Q1 and Q2 respectively through current limiting resistors R2 and R3.
Referring to FIG. 2, there is shown a circuit diagram of another conventional electronic ballast circuit for discharge lamps. In operation, upon application of an input AC voltage, the input AC voltage is full wave-rectified by diodes D11-D14 and then smoothed by a condenser C11. As a result, a DC voltage is supplied from the condenser C11. The DC voltage from the condenser C11 is supplied to a base of a transistor Q11 through a turning-on resistor R11, thereby causing the transistor Q11 to be turned on. Because of the turning-on of the transistor Q11, a current flows through a current limiting transformer T11, a primary winding L41 of a transformer T4 and the transistor Q11.
At this time, the transistor Q11 and a transistor Q12 are alternately turned on by a voltage induced in a third winding L43 of the transformer T4, thereby causing a voltage to be induced in a fifth winding L45 of the transformer T4 in proportion to voltages induced in the primary winding L41 and a secondary winding L42 of the transformer T4. The voltage from the fifth winding L45 of the transformer T4 is supplied to the discharge lamp through condensers C14 and C15, resulting in a discharging operation.
FIG. 3A is a waveform diagram of a collector-emitter voltage of the transistor Q11, FIG. 3B is a waveform diagram of a base current of the transistor Q11 and FIG. 3C is a waveform diagram of a base voltage of the transistor Q11.
By the way, in order to drive the transistors Q11 and Q12 more stably, a voltage is supplied to the bases of the transistors Q11 and Q12 separately from the voltage induced in the third winding L43 of the transformer T4. This separate voltage is supplied to the bases of the transistors Q11 and Q12 in a different manner from that in FIG. 1. Namely, a voltage is induced in a fourth winding L44 of the transformer T4 as the primary winding L41 and the secondary winding L42 of the transformer T4 are driven. The voltage from the fourth winding L44 of the transformer T4 is full wave-rectified by a diode D15 and then smoothed by a condenser C12. As a result, a DC voltage is supplied from the condenser C12. The DC voltage from the condenser C12 is supplied to the bases of the transistors Q11 and Q12 respectively through current limiting resistors R13 and R14.
However, in the conventional electronic ballast circuits, in order to stabilize the transistors of the inverter, the base driving circuit is provided with the transformer, the bridge diode, the condenser and the resistors as shown in FIG. 1 or the diode, the condenser and the resistors as shown in FIG. 2, resulting in a degradation in the efficiency of the ballast circuits and an increase in the cost.